Most backpacks have no frame. They consist of little more than a container of pliant material to which shoulder straps are affixed. Day packs, knapsacks, book bags, bike bags, and the like, are examples.
Backpacks intended for carrying specifically identified loads often have frames shaped to accommodate that load. One of the most common is a backpack for carrying children. In that example, the frame and the pack, the container, form a sling or seat in which the child is seated while being carried.
On the other hand, general purpose packbacks which are to be capable for use in carrying heavy, bulky loads have a frame shaped to permit distribution of the load. Such frames usually comprise side bars interconnected at their upper and lower ends by cross-members to form a generally rectangular structure. The pack is made of pliant material and is suspended from the upper crossmember or the side bars or both cross-member and side bars.
An important feature of such a structure is that the element that rests against the wearer's back is separated physically from the walls of the pack container. The contents of the pack container are likely to cause depressions and bulges and protrusions in its forward side that would cause considerable discomfort if permitted to engage the pack carrier's back. Instead, a back rest is stretched taut across the side rails of the backpack frame in a region of the frame at which it, rather than the pack container, will rest upon the wearer's back. The shoulder straps of the conventional, framed backpack extend from an upper crossbar to respectively associated points on the side bars of the frame.
The result of the assemblage of back rest, shoulder straps, frame and pack container is an apparatus which is efficient for the performance of its primary task. A conditioned wearer can use it to carry heavy loads for long periods over difficult terrain. However, once removed from the wearer's back, it is awkward, will not readily stand with the pack container upright, and is thoroughly inconvenient.